Internet DRAFT - draft-tschofenig-lamps-nonce-cmp-est
draft-tschofenig-lamps-nonce-cmp-est
LAMPS H. Tschofenig
Internet-Draft H. Brockhaus
Intended status: Standards Track Siemens
Expires: 3 September 2024 2 March 2024
Nonce-based Freshness for Remote Attestation in Certificate Signing
Requests (CSRs) for the Certification Management Protocol (CMP) and for
Enrollment over Secure Transport (EST)
draft-tschofenig-lamps-nonce-cmp-est-01
Abstract
Certificate Management Protocol (CMP) and Enrollment over Secure
Transport (EST) define protocol messages for X.509v3 certificate
creation and management. Both protocol provide interactions between
client systems and PKI management entities, such as a Registration
Authority (RA) and a Certification Authority (CA).
CMP and EST allow an RA/CA to request additional information it has
to provide in a certification request. When an end entity places
attestation Evidence in a Certificate Signing Request (CSR) it may
need to demonstrate freshness of the provided Evidence. Attestation
technology today often accomplishes this task via the help of nonces.
This document specifies how nonces are provided by an RA/CA to the
end entity for inclusion in Evidence.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on 3 September 2024.
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Copyright Notice
Copyright (c) 2024 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology and Requirements Language . . . . . . . . . . . . 4
3. Conveying a Nonce in CMP . . . . . . . . . . . . . . . . . . 5
4. Conveying a Nonce in EST . . . . . . . . . . . . . . . . . . 6
5. Nonce Processing Guidelines . . . . . . . . . . . . . . . . . 8
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
7. Security Considerations . . . . . . . . . . . . . . . . . . . 9
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
8.1. Normative References . . . . . . . . . . . . . . . . . . 10
8.2. Informative References . . . . . . . . . . . . . . . . . 11
Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
Several protocols have been standardized to automate the management
of certificates, such as certificate issuance, CA certificate
provisioning, certificate renewal and certificate revocation.
The Certificate Management Protocol (CMP) [I-D.ietf-lamps-rfc4210bis]
defines protocol messages for X.509v3 certificate creation and
management. CMP provides interactions between end entities and PKI
components, such as a Registration Authority (RA) and a Certification
Authority (CA).
Enrollment over Secure Transport (EST) [RFC7030][RFC8295] is another
certificate management protocol, which provides a sub-set of the
features offered by CMP.
An end entity requesting a certificate from a Certification Authority
(CA) may wish to offer believable claims about the protections
afforded to the corresponding private key, such as whether the
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private key resides on a hardware security module or the protection
capabilities provided by the hardware, and claims about the platform
itself.
To convey these claims in Evidence, as part of remote attestation,
the remote attestation extension [I-D.ietf-lamps-csr-attestation] has
been defined. It describes how to encode Evidence produced by an
Attester for inclusion in Certificate Signing Requests (CSRs), and
any certificates necessary for validating it.
A Verifier or Relying Party might need to learn the point in time an
Evidence has been produced. This is essential in deciding whether
the included Claims can be considered fresh, meaning they still
reflect the latest state of the Attester.
Attestation technology available today, such as [TPM20] and
[I-D.tschofenig-rats-psa-token], often accomplish freshness with the
help of nonces. More details about ensuring freshness of Evidence
can be found in Section 10 of [RFC9334].
Since an end entity needs to obtain a nonce from the Verifier via the
Relying Party, this leads to an additional roundtrip. The CSR is,
however, a one-shot message. For this reason CMP and EST are used by
the end entity to obtain the nonce from the RA/CA.
CMP and EST conveniently offer a mechanism to request information
from the RA/CA prior to a certification request.
Once the nonce is obtained the end entity can issue an API call to
the Attester to request Evidence and passes the nonce as an input
parameter into the API call. The returned Evidence is then embedded
into a CSR and returned to the RA/CA in a certification request
message.
Figure 1 shows this interaction graphically. The nonce is obtained
in step (1) using the extension to CMP/EST defined in this document.
The CSR extension of [I-D.ietf-lamps-csr-attestation] is used to
convey Evidence to the RA/CA in step (2). The Verifier processes the
received information and returns an Attestation Result to the Relying
Party in step (3).
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.---------------.
| |
| Verifier |
| |
'---------------'
| ^ | (3)
| | | Attestation
| | | Result
(1) | | v
.------------. Nonce in .----|----|-----.
| | CMP or EST | | | |
| End |<-------------------+ | |
| Entity | | | |
| ^ |-------------->|---------' |
| | | Evidence | Relying |
| v | in CSR | Party (RA/CA) |
| Attester | (2) | |
| | | |
'------------' '---------------'
Figure 1: Architecture with Background Check Model.
The functionality of this document is described in two sections,
namely:
* Section 3 describes how to convey the nonce CMP, and
* Section 4 offers the equivalent functionality for EST.
2. Terminology and Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
The terms Attester, Relying Party, Verifier and Evidence are defined
in [RFC9334]. The terms end entity, certification authority (CA),
and registration authority (RA) are defined in [RFC5280].
We use the terms Certificate Signing Request (CSR) and certification
request interchangeably.
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3. Conveying a Nonce in CMP
Section 5.3.19.16 of [I-D.ietf-lamps-rfc4210bis] defines the general
request message (genm) and general response (genp). The NonceRequest
payload of the genm message, which is send by the end entity to
request a nonce, contains optional details on the length of nonce the
Attester requires. The NonceResponse payload of the genp message,
which is sent by the CA/RA in response to a request message by the
end entity, contains the nonce.
GenMsg: {id-it TBD1}, NonceRequestValue
GenRep: {id-it TBD2}, NonceResponseValue | < absent >
id-it-NonceRequest OBJECT IDENTIFIER ::= { id-it TBD1 }
NonceRequestValue ::= SEQUENCE {
len INTEGER OPTIONAL,
-- indicates the required length of the requested nonce
hint EvidenceHint OPTIONAL
-- indicates which Verifier to request a nonce from
}
id-it-NonceResponse OBJECT IDENTIFIER ::= { id-it TBD2 }
NonceResponseValue ::= SEQUENCE {
nonce OCTET STRING
-- contains the nonce of length len
-- provided by the Verifier indicated with hint
expiry Time OPTIONAL
-- indicates how long the Verifier considers the
-- nonce valid
}
Note: The EvidenceHint structure is defined in
[I-D.ietf-lamps-csr-attestation]. The hint is intended for an
Attester to indicate to the EST server which Verifier should be
invoked to request a nonce.
The use of the general request/response message exchange leads to an
extra roundtrip to convey the nonce from the CA/RA to the end entity
(and ultimately to the Attester inside the end entity).
The end entity MUST construct a NonceRequest request message to
trigger the RA/CA to transmit a nonce in the response.
[Open Issue: Should request message indicate the remote attestation
capability of the end entity rather than relying on "policy
information"? This may also allow to inform the CA/RA about the type
of attestation technology/technologies available to the end entity.]
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If the end entity supports remote attestation and the policy requires
Evidence in a CSR to be provided, the RA/CA issues an NonceResponse
response message containing a nonce.
Figure 2 showns the interaction graphically.
End Entity RA/CA
========== =============
-->>-- NonceRequest -->>--
Verify request
Generate nonce*
Create response
--<<-- NonceResponse --<<--
(nonce, expiry)
Generate key pair
Generate Evidence*
Generate certification request message
-->>-- certification request -->>--
(+Evidence including nonce)
Verify request
Verify Evidence*
Check for replay*
Issue certificate
Create response
--<<-- certification response --<<--
Handle response
Store certificate
*: These steps require interactions with the Attester
(on the EE side) and with the Verifier (on the RA/CA side).
Figure 2: CMP Exchange with Nonce and Evidence.
4. Conveying a Nonce in EST
The EST client can request a nonce for its Attester. This function
is generally performed after requesting CA certificates and before
other EST functions.
The EST server MUST support the use of the path-prefix of "/.well-
known/" as defined in [RFC5785] and the registered name of "est".
Thus, a valid EST server URI path begins with
"https://www.example.com/.well-known/est". Each EST operation is
indicated by a path-suffix that indicates the intended operation.
The following operation is defined by this specificaion:
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+------------------------+-----------------+-------------------+
| Operation |Operation path | Details |
+========================+=================+===================+
| Retrieval of a nonce | /nonce | This section |
+------------------------+-----------------+-------------------+
The operation path is appended to the path-prefix to form the URI
used with HTTP GET or POST to perform the desired EST operation. An
example valid URI absolute path for the "/nonce" operation is
"/.well-known/est/nonce".
An EST client uses a GET or a POST depending on whether parameters
are included:
* A GET request MUST be used when the EST client does not want to
convey extra parameters.
* A POST request MUST be used when parameters, like nonce length or
a hint about the verification service, are included in the
request.
+-------------------+---------------------------------+---------------+
| Message type | Media type(s) | Reference |
| (per operation) | | |
+===================+=================================+===============+
| Nonce Request | N/A (for GET) or | This section |
| | application/json (for POST) | |
+===================+=================================+===============+
| Nonce Response | application/json | This section |
| | | |
+===================+=================================+===============+
To retrieve a nonce using a GET, the EST client would use the
following HTTP request-line:
GET /.well-known/est/nonce HTTP/1.1
To retrieve a nonce by specifying the size of the requested nonce
(and/or by including a hint about the Verification service) a POST
message is used, as shown below:
POST /.well-known/est/nonce HTTP/1.1
Content-Type: application/json
{
"len": 8,
"hint": "https://example.com"
}
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The payload in a POST request MUST be of content-type of
"application/json" and MUST contain a JSON object [RFC7159] with the
member "len" and/or "hint". The value of the "len" member indicates
the length of the requested nonce value in bytes. The "hint" member
contains either be a rfc822Name, a dNSName, a uri, or a test value
(based on the definition in the EvidenceHint structure from
[I-D.ietf-lamps-csr-attestation]).
The EST server MAY request HTTP-based client authentication, as
explained in Section 3.2.3 of [RFC7030].
If the request is successful, the EST server response MUST contain a
HTTP 200 response code with a content-type of "application/json" and
a JSON object [RFC7159] with the member nonce. The expiry member is
optional and indicates the time the nonce is considered valid. After
the expiry time is expired, the session is likely garbage collected.
Below is a non-normative example of the response given by the EST
server:
HTTP/1.1 200 OK
Content-Type: application/json
{
"nonce": "MTIzNDU2Nzg5MDEyMzQ1Njc4OTAxMjM0NTY3ODkwMTI=",
"expiry": "2031-10-12T07:20:50.52Z"
}
[Open Issue: Should we also register a content type for use with EST
over CoAP where the nonce and the expiry field is encoded in a CBOR
structure?]
5. Nonce Processing Guidelines
When the RA/CA is requested or configured to transmit a nonce to an
end entity then it interacts with the Verifier. The Verifier is,
according to the IETF RATS architecture [RFC9334], "a role performed
by an entity that appraises the validity of Evidence about an
Attester and produces Attestation Results to be used by a Relying
Party." Since the Verifier validates Evidence it is also the source
of the nonce to check for replay.
The nonce value MUST contain a random byte sequence whereby the
length depends on the used remote attestation technology. Since the
nonce is relayed with the RA/CA, it MUST be copied to the respective
structure, as described in Section 4 and Section 3, for transmission
to the Attester.
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For example, the PSA attestation token
[I-D.tschofenig-rats-psa-token] supports nonces of length 32, 48 and
64 bytes. Other attestation technologies use nonces of similar
length. The assumption in this specification is that the RA/CA have
out-of-band knowledge about the required nonce length required for
the attestation technology used by the end entity. The nonces of
incorrect length will cause the remote attestation protocol to fail.
When the end entity receives the nonce it MUST use it, if remote
attestation is available and supports nonces. It is better for an
RA/CA to be aggressive in sending a nonce, at least where there is a
reasonable chance the client supports remote attestation and the
client should be allowed to ignore the nonce if it either does not
support it or cannot use it for policy reasons.
While the semantics of the attestation API and the software/hardware
architecture is out-of-scope of this specification, the API will
return Evidence from the Attester in a format specific to the
attestation technology utilized. The encoding of the returned
evidence varies but will be placed inside the CSR, as specified in
[I-D.ietf-lamps-csr-attestation]. The software creating the CSR will
not have to interpret the Evidence format - it is treated as an
opaque blob. It is important to note that the nonce is carried in
the Evidence, either implicitly or explicitly, and it MUST NOT be
conveyed in CSR structures outside the Evidence payload.
The processing of the CSR containing Evidence is described in
[I-D.ietf-lamps-csr-attestation]. Note that the issued certificates
does not contain the nonce, as explained in
[I-D.ietf-lamps-csr-attestation].
6. IANA Considerations
TBD:
7. Security Considerations
This specification defines how to obtain a nonce via CMP and EST and
assumes that the nonce does not require confidentiality protection
without impacting the security property of the remote attestation
protocol. [RFC9334] defines the IETF remote attestation architecture
discusses nonce-based freshness in great detail.
Section 8.4 of [I-D.ietf-rats-eat] places randomness and privacy
requirement on the generation of the nonce for use with an Entity
Attestation Token (EAT). These requirements have been adopted by
attestation technologies, such as the PSA attestation token
[I-D.tschofenig-rats-psa-token] and are of general utility:
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* The nonce MUST have at least 64 bits of entropy.
* To avoid the conveyance of privacy-related information in the
nonce, it should be derived using a salt that originates from a
true and reliable random number generator or any other source of
randomness.
Since each specification of an attestation technology offers guidance
for replay protection with nonces (and other techniques) this
document needs to defer specific guidance to the respective
specifications.
For the use of Evidence in a CSR the security considerations of
[I-D.ietf-lamps-csr-attestation] are relevant to this document.
8. References
8.1. Normative References
[I-D.ietf-lamps-csr-attestation]
Ounsworth, M., Tschofenig, H., and H. Birkholz, "Use of
Remote Attestation with Certificate Signing Requests",
Work in Progress, Internet-Draft, draft-ietf-lamps-csr-
attestation-08, 1 March 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-lamps-
csr-attestation-08>.
[I-D.ietf-lamps-rfc4210bis]
Brockhaus, H., von Oheimb, D., Ounsworth, M., and J. Gray,
"Internet X.509 Public Key Infrastructure -- Certificate
Management Protocol (CMP)", Work in Progress, Internet-
Draft, draft-ietf-lamps-rfc4210bis-08, 1 March 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-lamps-
rfc4210bis-08>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/rfc/rfc2119>.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<https://www.rfc-editor.org/rfc/rfc5280>.
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[RFC5785] Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known
Uniform Resource Identifiers (URIs)", RFC 5785,
DOI 10.17487/RFC5785, April 2010,
<https://www.rfc-editor.org/rfc/rfc5785>.
[RFC7030] Pritikin, M., Ed., Yee, P., Ed., and D. Harkins, Ed.,
"Enrollment over Secure Transport", RFC 7030,
DOI 10.17487/RFC7030, October 2013,
<https://www.rfc-editor.org/rfc/rfc7030>.
[RFC7159] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March
2014, <https://www.rfc-editor.org/rfc/rfc7159>.
[RFC8295] Turner, S., "EST (Enrollment over Secure Transport)
Extensions", RFC 8295, DOI 10.17487/RFC8295, January 2018,
<https://www.rfc-editor.org/rfc/rfc8295>.
8.2. Informative References
[I-D.ietf-rats-eat]
Lundblade, L., Mandyam, G., O'Donoghue, J., and C.
Wallace, "The Entity Attestation Token (EAT)", Work in
Progress, Internet-Draft, draft-ietf-rats-eat-25, 15
January 2024, <https://datatracker.ietf.org/doc/html/
draft-ietf-rats-eat-25>.
[I-D.tschofenig-rats-psa-token]
Tschofenig, H., Frost, S., Brossard, M., Shaw, A. L., and
T. Fossati, "Arm's Platform Security Architecture (PSA)
Attestation Token", Work in Progress, Internet-Draft,
draft-tschofenig-rats-psa-token-22, 21 February 2024,
<https://datatracker.ietf.org/doc/html/draft-tschofenig-
rats-psa-token-22>.
[RFC9334] Birkholz, H., Thaler, D., Richardson, M., Smith, N., and
W. Pan, "Remote ATtestation procedureS (RATS)
Architecture", RFC 9334, DOI 10.17487/RFC9334, January
2023, <https://www.rfc-editor.org/rfc/rfc9334>.
[TPM20] Trusted Computing Group, "Trusted Platform Module Library
Specification, Family 2.0, Level 00, Revision 01.59",
November 2019,
<https://trustedcomputinggroup.org/resource/tpm-library-
specification/>.
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Appendix A. Acknowledgments
We would like to thank Russ Housley, Thomas Fossati, Watson Ladd,
Ionut Mihalcea and Carl Wallace for their review comments.
Authors' Addresses
Hannes Tschofenig
Siemens
Email: hannes.tschofenig@siemens.com
Hendrik Brockhaus
Siemens
Email: hendrik.brockhaus@siemens.com
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